1. Field of the Invention
The present application relates to a method and apparatus employed to thermoform thermoplastic foam materials such as polystyrene foam utilizing a matched mold thermoforming operation which employs both a plug assist and vacuum draw during the molding step.
2. Brief Description of Prior Art Practices
In the past, matched mold thermoforming has been commonly employed to form articles from preformed thermoplastic sheet material, which sheet is initially formed utilizing well known thermoplastic extrusion techniques. The sheet is subsequently preheated and placed between male and female mold halves, which, as they close, press and form the sheet into the desired product shape. Obviously, in such an operation, the material distribution of the formed product will depend upon the shapes of the mold halves.
An alternate forming arrangement which may be employed to thermoform plastic sheet includes vacuum thermoforming. A vacuum is applied beneath the preheated sheet to be formed causing atmospheric pressure to push the sheet down into contact with the mold. As the sheet contacts the mold it cools and sets in the desired configuration. Usually those areas of the sheet material which reach the vacuum mold member last are the thinnest having been drawn to a greater extent than the remainder of the material being formed.
Other prior art thermoforming techniques include a two-stage thermoforming technique whereby, utilizing a plug member, a preheated plastic sheet is only partially preformed into a desired configuration and, after the preforming step, the thermoforming step is completed whereby the matched mold members come together to form the desired finished article. U.S. Pat. No. 3,825,166 discloses such a forming method.